Hello Austin,

Sand should only be used to level lawns when the substrate soil is sandy as well. https://hgic.clemson.edu/factsheet/topdressing-a-home-lawn/

If you are on a soil that is mostly clay, similar soil should be used to level your lawn. There are three ways that are effective in levelling a bumpy lawn. If you have large, low areas, you can peel the grass back or cut it out in uniform thickness strips. You can then fill in the low area to within the strips’ thickness and replace the strips of grass you cut. If low areas are smaller or very numerous, you can use soil similar to your clay substrate and add thin layers over the low area every week of two. The objective is never to completely cover the grass plants. This will suffocate and kill the crown. However, if you continue to add thin layers of soil, allowing the crown to reestablish to the new height each time, you can eventually fill in the low areas: it is just a slower process. This is similar to topdressing. For a clay substrate, topsoil or compost is an acceptable topdressing medium. Topdressing is generally most effective when it is accompanied by core aerating. Finally, you can completely fill in all the low areas with soil similar to your substrate and reseed the filled in spots. Completely covering the crowns will smother them, so the grass will not reestablish higher in the soil, so you will need to reseed the areas.

Here are some references on top dressing

https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3710.pdf

https://nwdistrict.ifas.ufl.edu/hort/2016/03/11/why-do-people-apply-sand-over-their-lawns/

If lawn is just bumpy and does not have localized low spots, rolling may be a solution. Effective rolling of bumpy lawns is addressed in this article:

https://yardandgarden.extension.iastate.edu/how-to/bumpy-rough-lawns

Hope this helps.